The order
Gymnosomata (often referred to as "sea angels") includes specialized pelagic predators that lack shells
and use an expanded portion of the foot for swimming. Most have
elongate bodies.
In older
literature, they are often included with the Thecosomata in the order
Pteropoda (known as "sea butterflies").

Hydromyles globulosa
(Rang, 1825) in the family HydromylidaeNotobranchaea inopinata
Pelseneer, 1887 in the family NotobranchaeidaePneumodermopsis ciliata
(Gegenbaur, 1855) in the family PneumodermatidaeThliptodon diaphanus
(Meisenheimer, 1903) in the family Clionidae